Conferencing, wherein several telephone users talk to each other, is a popular feature in private automatic branch exchange (PABX) systems. Generally in these systems voices are converted to pulse code modulated (PCM) signals, which PCM signals are transmitted serially on a PCM data bus. In prior conferencing systems it is necessary to convert the PCM signals into the analog domain or into linear binary signals. In one of the latest conferencing systems a chip converts the incoming serial PCM signals into parallel form. The parallel data is then converted to 14-bit linear coded binary words from the 8-bit non-linear format in which it was originally coded. The linear-coded voice data from more than one source is then summed in a standard adder circuit to create the composite conference signal. Following this addition, a circuit verifies that the sum does not overflow line capacity. If the sum is too high a circuit automatically attenuates the conference signal to a level the line can accept. The signal is then reconverted into the original PCM-coded format and then converted from parallel form into a serial PCM bit stream. The conference signal can then be supplied back to the various conferencing channels.
These prior art conferencing systems are extremely complicated and expensive. Also, the various conversions and the attenuation introduce distortion into the signals.